1. Field of the Invention
The present invention relates to recording information by thermo-optical means.
2. Description of the Prior Art
This recording technique is based on the use of a heat-sensitive structure possessing a reference surface the optical characteristics of which are locally modified. These modifications are readable by optical means and result from interaction caused by a focused laser beam which is scanned over the reference surface and the intensity of which is modulated according to the information to be transcribed.
The thermo-optical recording technique offers the advantage of requiring no processing subsequent to exposure of the photo-sensitive surface, so that the heat-sensitive structure may be protected as soon as it is fabricated by means of an envelope permeable to the writing and reading radiation.
Moreover, the fact that reading may be carried out immediately after writing data facilitates the use of an information medium of this kind as a high information density backing store.
Thermo-optical recording is based on the conversion into heat of as high a proportion as possible of the incident radiant energy.
This presupposes that one extremely thin element of the heat-sensitive structure is capable of absorbing the incident radiation. The heat generated by the impact of the light beam is intended to procure the required modification of optical characteristics by virtue of the temperature rise associated with the generation of heat. For reasons of efficiency and resolution, the transient temperature rise must remain confined within the immediate proximity of the center of the area of impact of the writing beam. When the temperature rise at the center reaches a predefined threshold value it initiates the thermal phase proper of the writing process, which may make use of a phenomenon of ablation, of coalescence, of deformation or of change in texture. In this way it is possible to distinguish optically the areas of the heat-sensitive structure in which the thermal phenomenon has occurred from those which have retained their initial appearance.
The present invention is more particularly directed to depositing a continuous metal film with a thickness between a few tens and a few hundreds of Angstroms, this layer constituting in a heat-sensitive structure the incident radiation collecting element which is the source of heat emission; its shape or texture may be changed because of its own transient temperature rise and/or the heating effect which it communicates to an underlying element with which it cooperates. The optically absorbent metal film may typically form part of a heat-sensitive structure entailing the formation of surface irregularities such as craters or bubbles, or structures which retain a virtually smooth appearance but where optical readability results from an optically detectable change of texture.
French patent application No. 80/01423 filed Jan. 23, 1980 by THOMSON-CSF and directed to a "THERMO-OPTICAL INFORMATION WRITING PROCESS AND INFORMATION MEDIUM FOR USE OF THIS PROCESS" describes a thermo-optical writing process which consists in causing a localised loss of adherence at the point of impact of a writing beam between an optically absorbent metal layer and an underlying organic layer of low thermal diffusivity. This loss of adherence is associated with the release of gases which, by expanding, deform the metal layer without rupturing it. French patent application No. 76/31867 filed Oct. 22, 1976 by Thomson Brandt and directed to "INFORMATION RECORDING MEDIUM AND PROCESS FOR RECORDING INFORMATION ON SAME" describes a similar structure with two layers in which the release of gases can cause rupture of the metal layer. Both these documents suggest depositing the metal layer by means of the Joule effect, that is to say by evaporating metals such as gold and chromium in a strong vacuum. U.S. Pat. No. 4,414,273 suggests fabricating a heat-sensitive information medium comprising a heat-sensitive metal layer. It suggests a large number of metals which may be combined including gold and chromium and makes reference to numerous deposition techniques for obtaining thicknesses of the order of 300 to 1,500 Angstroms. Sputtering and vapor phase deposition are mentioned in this document.
Thermo-optical exposure of an information medium being intended to employ the impact of a beam of radiation to address a heat-sensitive area, the change of shape or texture must produce a lasting trace in the reference surface scanned by the beam and this trace must be as clear and fine as possible. The function of the metal layer is thus to absorb the radiation, to oppose spreading out of the heat over the surface and, when the change of shape or texture has taken place, to participate in the development of an impression which can be read accurately not only in the short term but also in the long term.
As it is known that the thickness of a thin metal layer is one factor determining its absorbent properties and thermal diffusivity, it is important to determine the composition and the deposition technique which offer the greatest latitude for exposure without losing sight of the required sensitivity, the manufacturing yield and the long-term stability of the heat-sensitive structure. In practice, one seeks to obtain for thicknesses up to 300 Angstroms high absorbency associated with low thermal diffusivity. As measuring thermal diffusivity is a delicate matter, it may be deduced from other physical quantities, namely the specific mass and the thermal conductivity. In characterizing a metal deposit qualitatively in terms of thermal conductivity, reliance may be placed on the electrical conductivity, since there is a correlation between these two quantities and the second is easier to measure.
The vacuum evaporation technique using a compound based on a precious metal and an additive metal to form a metal film on the surface of an organic material has been suggested in French patent application No. 80/01423 as constituting a structure element appropriate for writing thermo-optically. Gold cited by way of example makes it possible to achieve resistance to oxidation.
An alloy containing 20% chromium and 80% gold (by weight) in thicknesses from 40 to 100 Angstroms has made it possible to achieve high optical absorbency and very significant qualities of hardness and ductility for etching bubbles with an incident laser power of only eight milliwatts. Vacuum evaporation has its disadvantages, however.
The crucible in which the metal constituents are heated is governed by GIBBS' phase law, as a result of which the composition of the deposit is only tenuously related to the content of the crucible. Another disadvantage is associated with the kinetics of the metal particles on approaching the layer support which means that the sites occupied by the first particles attached to the support tend to capture further particles, preventing them from giving rise to numerous nucleation points.
There results a deposit in the form of crystalline aggregates of relatively coarse texture, generally resulting in a relatively low elastic limit. Moreover, vacuum evaporation tends to favor the existence of oxidized abutting regions the thickness of which is non-negligible relative to the overall thickness of the film deposited.
In order to alleviate these disadvantages, the invention proposes to deposit on a support a film of which the initial metal constituents are arranged on a target. This target, placed facing the support, is subject to intense ionic bombardment capable of conferring on the metal atoms extracted from the target sufficient energy to develop on the surface of the support a polycrystalline aggregate having a texture significantly finer than that produced by high vacuum evaporation deposition. This cathode sputtering deposition is effected at reduced pressure in a vacuum enclosure containing a rare gas. Means for magnetically confining the bombardment of the target are provided to permit collection of atoms as close as possible to the target and thus to favor obtaining a multiplicity of nucleation sites, which limits the growth of the grains.